High pressure metal vapor discharge lamp



Dec. 28, 1965 A. c. GREEN 3,226,597

HIGH PRESSURE METAL VAPOR DISCHARGE LAMP Filed Sept. 4, 1963 lnveen tor:ALwin C. Green 8 His Air-L- w-nel United States Patent 3,226,597 HIGHPRESSURE METAL VAPOR DISCHARGE LAMP Alwin C. Green, Hiram, Ohio,assiguor to General lectric Company, a corporation of New York FiledSept. 4, 1963, Ser. No. 306,476 Claims. (Ci. 31560) This inventionrelates to metallic vapor lamps using an arc discharge in mercury andmetal halide vapors to produce visible light, and is more particularlyconcerned with features having to do with the starting thereof.

The mercury arc lamp has achieved commercial acceptance by virtue of itslong life and reasonably good efficiency in the range of 50 to 60 lumensper arc watt. However, it suffers the disadvantage of relatively poorcolor rendition due to the bluish-green quality of its light, and alsoits efficiency is appreciably below the 70 to 80 lumens per arc wattrange of the ordinary fluorescent lamp. A radical improvement in bothcolor rendition and efficiency may be achieved by adding to the mercuryone or more vaporizable metal halides under proper control of loading,temperature and pressure, the preferred metal halide additive beingsodium iodide, optionally with thallium iodide. Such improved lamps aredescribed and claimed in copending application Serial No. 84,068 ofGilbert H. Reiling, filed Jan. 23, 1961, entitled Gaseous ElectricDischarge Lamps and assigned to the same assignee as the presentinvention. For convenience, such lamps will hence forth be referred toherein as mercury metal halide lamps.

In its general construction and appearance, the mercury metal halidelamp resembles the conventional high pressure mercury vapor lamp. Itcomprises a quartz arc tube mounted within a glass outer jacket having ascrew base at one end. The are tube contains a quantity of mercury andmetal halide such as sodium iodide along with an inert gas such as argonfor starting purposes. Thermionic main electrodes are provided at theends of the arc tube, and an auxiliary starting electrode is locatedadjacent one of the main electrodes in order to facilitate starting, andis connected through a current limiting resistor to the other mainelectrode. In the case of the mercury metal halide lamp, the spacebetween the arc tube and the outer jacket is evacuated in order toreduce loss of heat from the arc tube to assure the required arc tubewall temperature. The ends of the arc tube are also preferably providedwith a heat reflecting coating in order to assure the desiredtemperature at the ends.

The presence of the starting electrode gives rise to a troublesomecondition which appears to be due to electrolysis of the metal halide.In the case of sodium iodide for instance, the salt at the beginning ofoperation is in liquid form and may electrically connect the startingelectrode to the adjacent main electrode. When this happens, the saltdecomposes with formation of a dark de posit and this may result inrapid failure of the seals at that end of the lamp. The object of theinvention is to remedy this condition.

-Devitrification of the seals between starting and adjacent mainelectrode in high pressure mercury vapor lamps is well known, and occurswhen a unidirectional potential arises between the starting and theadjacent main electrode by reason of current rectification by thestarting electrode. The solution is taught in Patent 2,660,692, St.Louis et al., and consists in increasing the electron emission of thestarting electrode to the point where its conductivity is substantiallysymmetrical on both half cycles of the applied alternating current, asby placing the starting electrode so that its tip is heated in the arc.Although this technique is helpful in the mercury metal meters ofmercury, within the arc tube to ice halide lamp, it does not eliminatethe brown-black deposit around the starting electrode.

In accordance with my invention, I have found that a solution to theproblem resides in positively connecting the starting electrode to theadjacent main electrode after starting of the lamp so as to eliminateany possibility of potential difference between them. A practical andinexpensive way of accomplishing this is to provide a bimetal switchbetween the starting electrode and the adjacent main electrode. Theswitch is normally open and it is actuated by the heat of the lamp afterstarting and short-circuits the starting electrode to the adjacent mainelectrode. For maximum efiiciency, the bimetal switch may be arranged todisconnect the starting electrode from the current limiting resistor atthe same time as the short circuit connection to the adjacent mainelectrode is made.

For further objects and advantages and for a better understanding of theinvention, attention is now directed to the following detaileddescription of preferred embodiments to be read in conjunction with theaccompanying drawing. The features of the invention believed to be novelwill be more particularly pointed out in the appended claims.

In the drawing:

FIG. 1 is a side view of a mercury metal halide arc lamp embodying theinvention.

FIG. 2 is a similar view illustrating a variant of the invention.

Referring to the drawing and more particularly to FIG. 1, there is showna high pressure mercury vapor lamp 1 comprising an outer vitreousenvelope or jacket 2 of generally tubular form modified by a centralbulbous portion 3. It is provided at its outer end with a re-entrantstem 4 having a press 5 through which extend relatively stiff inleadwires 6,7 connected at their outer ends to the contacts of the usualscrew type base 8, namely the threaded shell 9 and the insulated centercontact 10.

The inner arc tube 12 is made of quartz or fused silica and has sealedtherein at opposite ends a pair of main arcing electrodes, 13 at thebase end and 14 at the outer end, plus an auxiliary starting electrode15 at the base end adjacent main electrode 13. The. electrodes haveinleads 16, 1'7 and 18, respectively, each including an intermediatethin foil section 19 hermetically sealed through full diameter pinchseals 21, 22 at the ends of the arc tube. Main electrodes 13, 14 eachcomprise a tungsten wire helix wrapped around a tungsten core wire.Activation of the main electrodes may be achieved by providing a smallelongated piece or sliver of thorium metal (not shown in the drawing)inserted between the core and the helix. Alternatively the electrodesmay be activated by a very thin layer of thorium. metal vacuum-depositedthereon. The are tube contains a quantity of mercury which is entirelyvaporized during operation of the lamp and which at such time exerts apressure in the range of l to 15 atmospheres. A quantity of sodiumiodide is provided in excess of that vaporized at the operatingtemperature of the arc tube which should be not less than 500 C. at anyplace. Since the lowest temperature is encountered at the ends, a heatreflective coating indicated by the speckling is applied to the ends ofthe arc tube and to the adjacent portions of the pinch seals. Also as aheat conservation measure, the interenvelope space is evacuated; gettermaterial provided in the channeled rings 23 may be flashed after sealingthe jacket in order to assure high vacuum. A rare gas such as argon isprovided at a low pressure, for instance at approximately 215millifacilitate starting and warm up.

The are tube is supported within the outer jacket by a frame or harpcomprising a single side rod 24 and rod portions'25 on the open side.The frame is welded at its base end to inlead wire 7 and has a pair oftransverse spring members 26, 27 fastened to its upper end WlhiCh bearagainst the outer tubular portion of the jacket to provide lateralsupport. The are tube is fastened to the frame by a pair of metal straps28, 29 which extend between rod 24 and rod portions 25 and encompass andclamp the pinch seals. The straps are spaced a distance away from theends of the arc chamber in order to limit the cooling effect and also toprevent dev-itrifi-cation of the quartz about the electrode inleads. Aconductor 30 threaded through an insulating glass sleeve 31 connectsinlead l6'ofn1ain electrode 13 to inlead wire dot the outer jacket whichin turn is connect-ed to base shell 9. A conductor 32 connects inlead 17of main electrode 14 to rod 24 through which circuit continuity isprovided, to center contact of the base.

The practice heretofore in connection with mercury vapor lamps has beento connect auxiliary starting electrode 15 permanently to main electrode14 at the opposite end of the arc tube by means of a current limitingresistor 33 connected between inlead 18 of the starting electrode andside rod 24 as illustrated in FIG. 1. Only a relatively low startingcurrent is needed to ionize the lamp, and the current limiting resistor33 may have a value of 40,000 ohms by way of example. When a linevoltage of 230 to 240 volts A.C. as indicated is applied across the lampterminals in series with a ballasting reactor 34, sub stantially thefull line voltage appears between main electrode 13 and auxiliaryelectrode 15, and thisis sufiicie-nt to start the lamp. In normaloperation, the voltage drop across the lamp may be approximately 100vol-ts and resistor 33 then limits the starting electrode current to Amilliampere or less.

In the mercury metal halide lamps having only the construction so fardescribed, a brown-black deposit has been observed to form around thestarting electrode which may cause violent failure of the lamp within ashort interval of time. The effect is most pronounced when the lamp isoperated with the starter electrode lower-most. The metal halide in theform of liquid sodium iodide then collects at the lowest point of thearc tube which is also the coolest point. The liquid salt mayelectrically connect thev starter and the adjacent main electrode. Underthese conditions, even a fraction of a milliampere of direct current isenough to cause electrolytic decomposition of the sodium iodide. Sealfailure then results within a short time, probably due to the attack ofthe sodium metal on the quartz. However, even when the lamp is operatedsuch that the starter electrode is uppermost, seal failure occurs. Underthese conditions, the failure appears to be more similar to electrolyticdevitrification failure in mercury vapor lamps, only aggravated by thepresence of the sodium iodide. The damage is most pronounced in thequartz about the starting electrode and the adjacent main electrode. Inaccordance with my invention, I have found, a solu tion to the foregoingproblem which consists in providing a thermal switch whichshort-circuits the starter electrode to the adjacent rnain electrodeafter the lamp has started. As illustrated in FIG. 1, the thermal switchmay consist of a strip of dissimilar metals'bent toa U-shape and havingone leg welded or otherwise fastened to inlead 16 of main electrode 13.At room temperature, the switch extremities lie close together. As thelamp warms up, the U-shaped piece opens out and the piece of molybdenumwire 36 welded across the free end of the bimetal strip engagesmolybdenum inlead 18 of the starting electrode. 1 have found it suitableto use a strip of bimetal having a thickness of approximately .010 inch,43 x 3 millimeters in dimensions, and formed to a U about /4 inch inoverall length. By way of example, in a 400-watt lamp the bimetal switchcloses 2 to 4 minutes after starting of the lamp and substantiallyeliminates the seal failure problem.

In the embodiment of FIG. 1, the bimetal switch by short-circuiting thestarter electrode to the adjacent main electrode, in effect leaves thestarting resistor 33 connected in parallel with the arc tube duringoperation of the lamp. This means that the arc voltage is applied at alltimes across the starting resistor and entails a wattage loss therein ofA to /2 watt duringoperation. This may be avoided according toembodiment in in FIG. 2 by connecting the starting resistor 33 betweenthe harp and a blind pin terminal 37 provided in the pinch seal of thearc tube. The bimetal is fastened to inlead 18 of the starting electrodeand its free end is disposed to move between blind pin 37 and inlead 16of main electrode 13. In the rest position of the bimetal at ambienttemperature, it engages blind pin 37 so that the starting electrode isconnected through the starting resistor torod 24. After the lamp hasstarted, the free end of the bimetal disengages itself from blind pin 37and engages inlead 16 of main electrode 13. Thus in operation after thelamp has warmed up, the starting resistor is disconnected from thesupply and wattage loss in it is avoided. At the same time, the startingelectrode is short-circnited to the adjacent main electrode as requiredin order to prevent electrolysis of the sodium iodide, or electrolyticdamage to the seals. I

While the invention has been described by reference to specificpreferred embodiments of same, the details of construction described areintended as exemplary and not in order to limit the invention there-toexcept insofar as included in the accompanying claims.

What I claim as new and desire to secure by Letters Patent of the UnitedStates is:

1. An electric discharge lamp comprising a vitreous outer jacketenclosing a quartz arc tube, said are tube having a pair of mainthermionic electrodes sealed therein at opposite ends and a startingelectrode located adjacent one of said main electrodes sealed therein atone end, an ionizable filling within said are tube comprising mercury, ametal halide, and an inert starting gas, means supporting said are tubewithin said outer jacket and providing circuit connections to said mainelectrodes, means including a current limiting resistor within said,outer jacket connecting said starting electrode to the main electrode atthe opposite end of the arc tube, and a thermal switch actuated by theheat of said are tube during normal operation and arranged toshort-circuit the starting electrode to the adjacent main electrode whenactuated.

2. An electric discharge lamp comprising a vitreous outer jacketenclosing a quartz arc tube, said are tube having a pair of mainthermionic electrodes sealed therein at opposite ends and a startingelectrode located 'adjacent one of said main electrodes sealed thereinat one end, an ionizable filling within said are tube comprisingmercury, sodium iodide, and an inert starting gas, means supporting saidare tube within said outer jacket and providing circuit connections tosaid main electrodes, means including a current limiting resistor withinsaid outer jacket connecting said starting electrode to the mainelectrode at the opposite end of the arc tube, and a thermal switchunactuated at room temperature andactuated by the heat of said are tubeduring operation of said lamp, said thermal switch being arranged toshort-circuit'the starting electrode to the adjacent mainelectrode whenactuated.

3. An electric discharge lamp comprising a vitreous outer. jacketenclosing a quartz arc tube, said are tube having a pair of mainthermionic electrodes sealed therein atopposite ends and a startingelectrode located adjacent one of said main electrodes sealed therein atone end, an ionizable filling within said are tube comprising mercury, ametalhalide, and an inert starting gas, means supporting said are tubewithin said outer jacket and providing circuit connections to said mainelectrodes, 21 current limiting resistor connected to the main electrodeat the opposite end of the arc tube, and a thermal switch within saidouter jacket, said thermal switch when unactuated at room temperatureproviding circuit continuity between said starting electrode and saidresistor, and when actuated by the heat of said arc tube during normaloperation providing circuit continuity between said starting electrodeand the adjacent main electrode.

4. An electric discharge lamp comprising a vitreous outer jacketenclosing a quartz arc tube having main thermionic electrodes supportedby inleads sealed therein at opposite ends, a starting electrodeadjacent one of the main electrodes supported by an inlead sealedthrough said are tube proximately to that of the adjacent mainelectrode, an ionizable filling within said arc tube comprising mercury,sodium iodide and an inert gas at a low pressure, a frame includingconductors supporting said arc tube within said outer jacket andproviding circuit connections to said main electrodes, a currentlimiting resistor connected between said starting electrode and saidframe, and a U-shaped bimetal strip fastened at one end to the inlead ofone of said starting electrode and the adjacent main electrode andhaving its free end disposed to engage the inlead of the other whenactuated by the heat of said are tube, whereby to shortcircuit thestarting electrode to the adjacent main electrode during normaloperation of said lamp,

5. An electric discharge lamp comprising a vitreous outer jacketenclosing a quartz arc tube having main thermionic electrodes supportedby inleads sealed therein at opposite ends, a starting electrodeadjacent one of the main electrodes supported by an inlead sealedthrough said arc tube proximately to that of the adjacent mainelectrode, an ionizable filling within said arc tube comprising mercury,sodium iodide and an inert gas at a low pressure, a frame includingconductors supporting said are tube within said outer jacket andproviding circuit connections to said main electrodes, a blind pinmounted on the starting electrode end of said are tube, a startingresistor connected between said blind pin and said frame, and a U-shapedbimetal strip fastened at one end to the inlead of said startingelectrode and having its free end movable between said blind pin and theinlead of the adjacent main electrode, said bimetal strip operating toengage said blind pin at room temperature whereby to connect thestarting electrode through said resistor to the opposite main electrode,and engaging the inlead of the adjacent main electrode at highertemperatures corresponding to the operating condition of said lampwhereby to short-circuit the starting electrode to the adjacent mainelectrode and leave said starting resistor disconnected.

No references cited.

JOHN W. HUCKERT, Primary Examiner. DAVID J. GALVIN, Examiner.

1. AN ELECTRIC DISCHARGE LAMP COMPRISING A VITREOUS OUTER JACKETENCLOSING A QUARTZ ARC TUBE, SAID ARC TUBE HAVING A PAIR OF MAINTHERMIONIC ELECTRODES SEALED THEREIN AT OPPOSITE ENDS AND A STARTINGELECTRODE LOCATED ADJACENT ONE OF SAID MAIN ELECTRODES SEALED THEREIN ATONE END, AN IONIZABLE FILLING WITHIN SAIDS ARC TUBE COMPRISING MERCURY,A METAL HALIDE, AND AN INERT STARTING GAS, MEANS SUPPORTING SAID ARCTUBE WITHIN SAID OUTER JACKET AND PROVIDING CIRCUIT CONNECTIONS TO SAIDMAIN ELECTRODES, MEANS INCLUDING A CURRENT LIMITING RESISTOR WITHIN SAIDOUTER JACKET CONNECTING SAID STARTING ELECTRODE TO THE MAIN ELECTRODE ATTHE OPPOSITE END OF THE ARC TUBE, AND A THERMAL SWITCH ACTUATED BY THEHEAT OF SAID ARC TUBE DURING NORMAL OPERATION AND ARRANGED TOSHORT-CIRCUIT THE STARTING ELECTRODE TO THE ADJACENT MAIN ELECTRODE WHENACTUATED.